Since the introduction of the first implantable pacemakers in the 1960's, there have been considerable advancements both in the field of electronics and the field of medicine, such that there is presently a wide assortment of commercially-available implantable medical devices. The class of implantable medical devices now includes not only pacemakers, but also implantable cardioverters, defibrillators, neural stimulators, and drug administering devices. Today's state-of-the-art implantable medical devices are vastly more sophisticated and complex than early pacemakers, capable of performing significantly more complex tasks. The therapeutic benefits of such devices have been well-proven.
As the functional sophistication and complexity of implantable medical devices has increased over the years, it has become increasingly more important for such devices to be equipped with a telemetry system for enabling them to communicate with an external unit.
For example, shortly after the introduction of the earliest fixed-rate, non-inhibited pacemakers, it became apparent that it would be desirable for a physician to non-invasively exercise at least some amount of control over the device, e.g., to turn the device on or off or adjust the fixed pacing rate, after implant. In early devices, one way the physician was able to have some control over implantable device operation was through the provision of a remotely and non-invasively actuable switch, such as a magnetic reed switch, in the implantable device. After implant, the reed switch would be actuated by placing a magnet over the implant site. Reed switch closure could then be used, for example, to alternately activate or deactivate the device. Alternatively, the fixed pacing rate of the device could be adjusted up or down by incremental amounts based upon the duration of reed switch closure. Many different schemes utilizing a reed switch to adjust parameters of implanted medical devices have been developed. See, for example, U.S. Pat. No. 3,311,111 to Bowers (one or more reed switches used to switch in and out resistors to control either the charging or discharging of an RC circuit to control the pulse rate); U.S. Pat. No. 3,518,997 to Sessions; U.S. Pat. No. 3,623,486 to Berkovits; U.S. Pat. No. 3,631,860 to Lopin (reed switch toggled to increment a counter for selecting from among four possible pacing rates); U.S. Pat. No. 3,738,369 to Adams et al.; U.S. Pat. No. 3,805,796 to Terry, Jr. (reed switch is repeatedly toggled to alter pulse repetition rate and output current of implanted pulse generator); and U.S. Pat. No. 4,066,086 to Alferness et al. (reed switch used to actuate a circuit for receiving radio-frequency signals).
As new, more advanced features are incorporated into implantable devices, it is typically necessary to convey correspondingly more information to the device relating to the selection and control of those features. For example, if a pacemaker is selectively operable in various pacing modes (identified using the well-known NASPE/BPEG pacemaker codes such as VVI, VDD, DDD, etc . . . ), it is desirable that the physician or clinician be able to non-invasively select a mode of operation. Similarly, if the pacemaker is capable of pacing at various rates, or of delivering stimulating pulses of varying energy levels, it is desirable that the physician or clinician be able to select, on a patient-by-patient basis, appropriate values for such variable operational parameters.
Even greater demands are placed upon the telemetry system in implantable devices having such advanced features as rate adaptation based upon activity sensing, as disclosed, for example, in U.S. Pat. No. 5,052,388 to Sivula et al. entitled "Method and Apparatus for Implementing Activity Sensing in a Pulse Generator. The Sivula et al. '388 patent, which describes an implantable device commercially embodied as the Medtronic Legend.TM. pulse generator, is hereby incorporated by reference herein in its entirety.
The information which may need to be communicated to the implantable device in today's state-of-the-art pacemakers includes: pacing mode, multiple rate response settings, electrode polarity, maximum and minimum pacing rates, output energy (output pulse width and/or output current), sense amplifier sensitivity, refractory periods, calibration information, rate response attack (acceleration) and decay (deceleration), onset detection criteria, and perhaps many other parameter settings.
The need to be able to communicate more and more information to implanted devices quickly rendered the simple reed-switch closure arrangement insufficient. Also, it has become apparent that it would also be desirable not only to allow information to be communicated to the implanted device, but also to enable the implanted device to communicate information to the outside world.
For diagnostic purposes, for example, it is desirable for the implanted device to be able to communicate information regarding its operational status to the physician or clinician. State of the art implantable devices are available which can even transmit a digitized ECG signal for display, storage, and/or analysis by an external device.
Various telemetry systems for providing the necessary communications channels between an external unit and an implanted device have been shown in the art. Telemetry systems are disclosed, for example, in the following U.S. Patents: U.S. Pat. No. 4,539,992 to Calfee et al. entitled "Method and Apparatus for Communicating With Implanted Body Function Stimulator"; U.S. Pat. No. 4,550,732 to Batty Jr. et al. entitled "System and Process for Enabling a Predefined Function Within An Implanted Device"; U.S. Pat. No. 4,571,589 to Slocum et al. entitled "Biomedical Implant With High Speed, Low Power Two-Way Telemetry"; U.S. Pat. No. 4,676,248 to Berntson entitled "Circuit for Controlling a Receiver in an Implanted Device"; U.S. Pat. No. 5,127,404 to Wyborny et al. entitled "Telemetry Format for Implanted Medical Device"; U.S. Pat. No. 4,211,235 to Keller, Jr. et al. entitled "Programmer for Implanted Device"; U.S. Pat. No. 4,374,382 to Markowitz entitled "Marker Channel Telemetry System for a Medical Device"; and U.S. Pat. No. 4,556,063 to Thompson et al. entitled "Telemetry System for a Medical Device".
Typically, telemetry systems such as those described in the above-referenced patents are employed in conjunction with an external programming/processing unit. One programmer for non-invasively programming a cardiac pacemaker is described in its various aspects in the following U.S. Patents to Hartlaub et al., each commonly assigned to the assignee of the present invention and each incorporated by reference herein in its entirety: U.S. Pat. No. 4,250,884 entitled "Apparatus For and Method Of Programming the Minimum Energy Threshold for Pacing Pulses to be Applied to a Patient's Heart"; U.S. Pat. No. 4,273,132 entitled "Digital Cardiac Pacemaker with Threshold Margin Check"; U.S. Pat. No. 4,273,133 entitled Programmable Digital Cardiac Pacemaker with Means to Override Effects of Reed Switch Closure"; U.S. Pat. No. 4,233,985 entitled "Multi-Mode Programmable Digital Cardiac Pacemaker"; and U.S. Pat. No. 4,253,466 entitled "Temporary and Permanent Programmable Digital Cardiac Pacemaker."
Aspects of the programmer that is the subject of the foregoing Hartlaub et al. patents (hereinafter "the Hartlaub programmer") are also described in U.S. Pat. No. 4,208,008 to Smith, entitled "Pacing Generator Programming Apparatus Including Error Detection Means" and in U.S. Pat. No. 4,236,524 to Powell et al., entitled "Program Testing Apparatus". The Smith '008 and Powell et al. '524 patents are also incorporated by reference herein in their entirety.
While the use of magnetic reed-switch closure alone for the communication of information to an implanted device has proven inadequate for programming all of the many programmable features of current state-of-the-art devices, many modem devices continue to incorporate a magnetic reed switch or other type of remotely-actuable switch in association with their telemetry systems. Often, as in the case of the Medtronic Activitrax.TM. and Spectrax.TM. pulse generators, for example, reed switch closure is required before radio-frequency programming signals from an external programmer will be accepted by the pulse generator. Such an arrangement provides a safeguard against accidental reprogramming of the pulse generator by spurious radio-frequency signals to which a pacemaker patient may be exposed.
In addition, pacemakers often enter a so-called "magnet mode" in response to reed switch closure. In conventional magnet mode, the pulse generator switches to an asynchronous fixed-rate pacing mode, where this fixed magnet mode pacing rate reflects the depletion level of the pulse generator's internal power supply (battery). Such operation is useful for a number of reasons. First, with the advanced programmable pacing and sensing features available with modern pulse generators, it can often be quite difficult for a physician or clinician to readily verify proper operation of the implanted device by simply looking at a surface EKG monitor. That is, it is often difficult to ascertain what events are being sensed by the device, how the device is responding to sensed events, and how the patient's heart is responding to the stimulating pulses generated by the pulse generator. When the pulse generator is pacing at its fixed, asynchronous magnet mode rate, however, it is much easier for the physician or clinician to determine, for example, whether the stimulating pulses have sufficient energy to exceed to patient's stimulation threshold.
Many pacemakers, for example the Medtronic Activitrax II, are further designed to modulate their magnet mode pacing rate according to the level of battery depletion. That is, the magnet mode pacing rate is reduced in proportion to the level of battery depletion. Thus, the physician can be made aware of the battery depletion and make an estimate of expected device longevity merely by affecting reed switch closure and observing the magnet mode pacing rate.
The Activitrax II also performs a "threshold margin test" upon initiation of its magnet mode of operation. In the threshold margin test, the pulse generator issues three stimulating pulses at an accelerated, asynchronous rate. The third of these three pulses has a reduced energy level relative to the programmed output energy level. The physician can observe on a surface EKG whether this reduced-energy third pulse has sufficient energy to capture the heart, in order to verify that the programmed output energy level includes an adequate stimulating threshold safety margin. After completion of the threshold margin test, the Activitrax II resumes asynchronous magnet mode pacing at a rate which reflects the level of battery depletion.
A pacemaker operable to perform a threshold margin test and then in an asynchronous magnet mode in response to reed switch closure as discussed above is described in greater detail in the above-referenced U.S. Pat. No. 4,273,132 to Hartlaub et al.
As those of ordinary skill in the art will appreciate, life-threatening heart rhythm disorders such as ventricular tachycardia (VT) or ventricular fibrillation (VF) can easily be activated with stimuli applied to cardiac tissue during the critical relative refractory period (T-wave)--"the vulnerable phase"--of each cardiac cycle. See, e.g., Modem Cardiac Pacing, Barold, ed., NY: Futura Publishing Co., (1985), pp. 522-543. See also, Leonard S. Dreifus, M.D., "Interrelationship of Ventricular Fibrillation and Cardiac Pacing" in Modem Cardiac Pacing: A Clinical Overview, Furman et al., eds., MD: Charles Press, (1975), pp. 245-260.
A pacing stimulus delivered during the atrial relative refractory period can also cause serious heart rhythm disorders, in particular, atrial flutter or even atrial fibrillation, in cases where atrial or dual-chamber pacemakers are used.
In view of the complications which may arise as a result of pacing the heart during its vulnerable phase, the inventor believes that it may be advantageous to provide for a variation of the conventional magnet mode of operation, wherein pacing in the vulnerable phase of the cardiac cycle is avoided.